The present invention relates generally to magnetic media for recording information, and, more particularly, to disc drives with magnetic head assemblies which record information in tracks on thin film discs.
The computer industry continually seeks to reduce size of computer components and to increase the speed at which computer components operate. To this end, it is desired to reduce the size required to magnetically record bits of information. It is concomitantly important to maintain the integrity of the information as size is decreased, and magnetic storage of information must be virtually 100% error free. Moreover, the methods used to reduce size, increase speed and maintain information integrity in computer components must be very reproducible in a manufacturing setting and must not be overly costly. The present invention seeks to address these goals in a disc drive.
Disc drives which magnetically record, store and retrieve information on disc-shaped media are widely used in the computer industry. A write transducer is used to record information on the disc, and a read transducer is used to retrieve information from the disc. The reading and writing processes may be performed by a single structure, i.e., a read-write transducer, or alternatively may be performed by separate structures. In either case, the read transducer and the write transducer are generally both located on a single magnetic head assembly. The magnetic head assembly may include an air bearing slider which suspends the magnetic head assembly relative to the rotating disc by "flying" off air on the disc surface.
The magnetic head assembly is mounted on the end of a support or actuator arm, which positions the head radially on the disc surface. If the actuator arm is held stationary, the magnetic head assembly will pass over a circular path on the disc known as a track, and information can be read from or written to that track. Each concentric track has a unique radius, and reading and writing information from or to a specific track requires the magnetic head to be located above the track. By moving the actuator arm, the magnetic head assembly is moved radially on the disc surface between tracks.
The disc drive must be able to differentiate between tracks on the disc and to center the magnetic head over any particular track. Most disc drives use embedded "servo patterns" of magnetically recorded information on the disc. The servo patterns are read by the magnetic head assembly to inform the disc drive of track location. Tracks typically include both data sectors and servo patterns. Each data sector contains a header followed by a data section. The header may include synchronization information to synchronize various timers in the disc drive to the speed of disc rotation, while the data section is used for recording data.
Each servo pattern typically includes a "gray code" and a "servo burst". The gray code indexes the radial position of the track such as through a track number, and may also provide a circumferential index such as a sector number. The servo burst is a centering pattern to precisely position the head over the center of the track. Each servo burst includes magnetic transitions on the inside of the track interleaved with magnetic transitions on the outside of the track. If the magnetic head is centered over the track, the signal read from the inside transitions will be equal and opposite to the signal read from the outside transitions. If the magnetic head is toward the inside of the track, the signal from the inside transitions will predominate, and vice versa. By comparing portions of the servo burst signal, the disc drive can iteratively adjust the head location until a zeroed position error signal is returned from the servo bursts indicating that the head is properly centered with respect to the track.
Servo patterns are usually written on the disc during manufacture of the disc drive, after the drive is assembled and operational. The servo pattern information, and particularly the track spacing and centering information, needs to be located very precisely on the disc. However, at the time the servo patterns are written, there are no reference locations on the disc surface which can be perceived by the disc drive. Accordingly, a highly specialized device known as a "servo-writer" is used during writing of the servo-patterns. Largely because of the locational precision needed, servo-writers are fairly expensive, and servo-writing is a time consuming process.
Most servo-writers operate using the disc drive's own magnetic head. The servo-writer takes precise positional references to properly position the heads in the disc drive for the writing of the servo patterns, and to properly space the tracks with respect to one another on the disc surface. For instance, the servo writer may have a physical position sensor which takes a positional reference from the axis of the drive spindle, and may have an optical position sensor which determines the location of the magnetic heads with respect to the axis of the drive spindle. With precise positioning of the magnetic head known, the magnetic head of the disc drive is used to write the servo pattern on the disc. The servo writer may also include a magnetic head which writes a clock track at an outer radius of the disc. Once written, servo patterns serve as the positional references on the disc surface used by the disc drive during the entire life of the disc drive. The servo patterns are used to properly center the head over the desired track prior to reading or writing any data information from or to that track.
One approach to avoid traditional servo-writing has been to injection mold or stamp servo patterns on a plastic substrate disc. The magnetic material layer is then applied at a consistent thickness over the entire disc surface, including the depressions and protrusions in the servo patterns. After the disc is mechanically fabricated (i.e., after all the layers are applied), a magnetic bias is recorded on the servo patterns. For instance, a first magnetic field may magnetically initialize the entire disc at a one setting. Then a second magnetic field, localized at the surface of the disc and perhaps provided by the magnetic head of the drive, is used to magnetize the protruding portions of the servo patterns relative to the depressions. Because the protrusions are closer than the depressions to the magnetic initialization, the magnetization carried by the protrusions may be different than the magnetization carried by the depressions. When read, the resulting disc servo patterns show magnetic transitions between the depressions and the protrusions. This approach, referred to as a PERM disc, is being pursued by the Sony Corp.
While servo patterns in PERM discs do not require much of the specialized servo-writing equipment otherwise necessary, other problems have arisen. The depressions in the disc surface have a detrimental effect on the flyability of the air bearing slider. Additionally, in traditional servo-patterns, the magnitude of the position error signal from the servo pattern is based on transitions from magnetism in one direction to magnetism in the opposite direction. Because the depressions in the PERM servo patterns make no significant contribution to the output signal, the resultant position error signal of the servo patterns is half that of a traditional servo pattern. In practice, perhaps due to imperfect saturation of the magnetic medium in the depressions, the resultant position error signal of PERM servo patterns has an even lower signal to noise ratio, typically around one-third that of the traditional servo signal. Other methods to reduce the cost of servo-writing without the drawbacks of the PERM disc are desired.